Ok, I have an odd situation that started on the 5th night of a 6 night trip that we just returned from. This was a 5th trip in the 2011 Carriage Cameo 31ksls fifth wheel RV and have never had this situation occur.
We shower in the RV with the bath door closed, the bathroom window open and the Fan Tastic vent fan running on exhaust. Of course, we turn water the on and off during the shower ritual.
So here is what happened. Upon turning on the shower to rinse off, I began the smell foul grey water. After the other halfs shower and investigating and experimenting the next night as we both took our showers, we determined that the odor was acutally entering thru the vent fan! I could not believe it or understand how so since the fan was running.
It seems that as the water drained from the shower pan and into the holding tank that odors were expelled out of the tank vent. That I can easily understand. What I can't understand is how the odors could come in through the vent running on exhaust.
There was absolutely no odor coming from the basement, from under the sink, the toilet, or from the shower pan. The smell was definately coming from the vent above.
The vent pipe on the roof is within about 3 inches from the Maxx Air II vent cover. We were camped on the beach with a steady breeze. But I stil can't understand how the odors come come in through the vent with the fan running. Any ideas? Thanks, Phillip

This may be a stretch but on an older TT we had, the toilet had a small pencil size vent that went from just under where the water exited to wash around bowl, all the way down under the slide valve. I guess it was a vent to the tank side of the toilet so any pressure in the tank would be released rather than blow "stuff" up into the bowl or worse, your face.
Maybe your commode has some kind of vent like this?
I've looked at newer commodes and have never seen a vent like this but maybe they're hidden better.

I installed one of these timers on my bath fan so I don't have to run it while I'm in the shower (much less drafty!). At least you wouldn't have to stand in there while the smell is coming. I just turn it on for 20~30 minutes afterward. Works great for other "jobs" too.
It's rated for 120V and works great for 12VDC items.

My guess is that the fan is causing such negative pressure in the bathroom with the door closed, it could be pulling Grey tank vaoors past the traps in the shower and/or the sink drain. It is possible.

If the fan was on 'exhaust' (pulling air from bathroom area and pushing it out) then the smell from the roof vent line was probably coming in thru the 'open' bathroom window..........and you smelled it at the fan area (concentrated as it was exhausted).

Odors would have a very hard time flowing upstream against the air flow from fan.
Otherwise the fan was not exhausting BUT pulling air from outside into bathroom.

I have a 2010 Cameo. The factory cut off the vent pipe more than an inch below the roof panels and the hole through the roof is almost as large as the vent cap. My smell comes in just like yours, but it comes in through the entire ceiling and especially strong via the 110 circuit breaker panel.

Unfortunately the gap means moist air is always venting into the roof insulation.

I tried to replace stock vent with the cyclone, but Cameo uses a different size vent pipe than most. The cyclones work great, and 4 years ago you could buy them in 2 pipe sizes, but I can't find but the one size now.

JohnD222 wrote:I have a 2010 Cameo. The factory cut off the vent pipe more than an inch below the roof panels and the hole through the roof is almost as large as the vent cap. My smell comes in just like yours, but it comes in through the entire ceiling and especially strong via the 110 circuit breaker panel.

Unfortunately the gap means moist air is always venting into the roof insulation.

I tried to replace stock vent with the cyclone, but Cameo uses a different size vent pipe than most. The cyclones work great, and 4 years ago you could buy them in 2 pipe sizes, but I can't find but the one size now.

Interesting! I am definately going to remove the vent cap and see what I can see. It's an odd situation for sure.
I know the odor was not coiming in the window, from under the sink, the drain traps, or toilet. We took turns over the coures of several showers sticking our noses in all these areas! lol. I could place by foot over the shower drain and let water build up. Once I removed my foot and the water starting draining, the smell started. As soon as the water stopped draining, the smell stopped.
Thanks for all the replies. I'm going to remove the vent cap and see what I find and report back. Thanks, Phillip

What I am going to say is just a guess, but here goes.
If your shower holding tank can not vent to the outside (clogged maybe),
then it must burp air back up into the shower stall when more shower water tries to enter the tank.
Bad venting would explain your problem.

I had a very similar situation thinking it was the shower holding tank.
At the time I kept the grey valves open. The problem went away when I started holding both shower and galley tanks until full. I think what was happening in my situation was that deposits were being left on the sides of the water channel in the bottom of the gray tanks which started downgrade over time. Holding the water allowed a clean dump of all this stuff. I actually think the galley was my problem with food deposits.